If it's a 2006 55 then I'd say it's reasonable price given the low miles and what's about on autotrader and what i paid for my 54 plate back in 2006.
problem is you probably want a good deal rather than a fair price! But cupras are hard to price because you can only really sell them to someone who knows what they are and that their heart is set on buying one, so dealers won't budge on price when someone like you turns up wanting one!
see if they'll throw in other extra's on top of the usual mats and tax, such as your next service free or a nice strut brace to jazz up your engine bay, or alloys refurbed if they're scuffed. anything which is expensive to you but cheaper for them to sort.